15.1 Causes and consequences of biodiversity loss

Threats to Biodiversity

Human-Induced Habitat Destruction

Habitat loss is the single biggest driver of biodiversity decline. It happens when natural environments are converted for human use through agriculture, urbanization, or deforestation.

• Habitat destruction leads to fragmentation, which isolates populations and shrinks their gene pools. Smaller, disconnected populations are more vulnerable to inbreeding and local extinction.
• Fragmentation also disrupts migration routes and breeding grounds, cutting species off from resources they depend on.
• A major example: roughly 17% of the Amazon rainforest has been cleared since the 1970s, largely for cattle ranching and soy production.

Overexploitation means harvesting species from the wild faster than they can reproduce. This includes overfishing, poaching, and the illegal wildlife trade.

• Atlantic cod populations off Newfoundland collapsed in the early 1990s due to decades of industrial overfishing. The fishery was closed in 1992 and still hasn't fully recovered.
• The passenger pigeon, once numbering in the billions across North America, was hunted to extinction by 1914.

Pollution from human activities degrades habitats and directly harms organisms.

• Agricultural runoff carries pesticides and excess fertilizers into waterways, triggering algal blooms and oxygen-depleted "dead zones" (like the one in the Gulf of Mexico, which covers roughly 15,000 $km^2$ in bad years).
• Plastic pollution entangles marine life and is ingested by animals like sea turtles and seabirds, often causing starvation or internal injury.
• Oil spills devastate coastal and marine ecosystems. The 2010 Deepwater Horizon spill released approximately 4.9 million barrels of oil into the Gulf of Mexico.

Climate Change and Anthropogenic Impacts

Climate change, driven by anthropogenic greenhouse gas emissions, is reshaping habitats worldwide.

• Rising temperatures force species to shift their geographic ranges toward the poles or to higher elevations. Species that can't move fast enough face decline.
• Phenological mismatches occur when warming disrupts the timing between interdependent species. For instance, if flowers bloom earlier but their pollinators haven't arrived yet, both suffer.
• Coral bleaching happens when elevated ocean temperatures cause corals to expel their symbiotic algae (zooxanthellae), often killing the coral. The Great Barrier Reef experienced mass bleaching events in 2016, 2017, 2020, and 2022.
• Ocean acidification results from the ocean absorbing excess $CO_2$, lowering pH. This makes it harder for organisms like oysters, corals, and pteropods to build calcium carbonate shells.
• Changing precipitation patterns intensify droughts, floods, and wildfire frequency in many regions.

Invasive species are organisms introduced (intentionally or accidentally) by human activities into ecosystems where they didn't evolve.

• Without natural predators or diseases to keep them in check, invasive populations can grow rapidly and outcompete native species for food and habitat.
• Burmese pythons, released or escaped from the pet trade, have established breeding populations in the Florida Everglades. They've caused severe declines in native mammal populations, including raccoons, opossums, and marsh rabbits.

Extinction occurs when the last individual of a species dies. It's irreversible, meaning that species' unique genetic information is permanently lost.

• Scientists estimate current extinction rates are 100 to 1,000 times higher than the natural background rate, leading many to call this the sixth mass extinction.
• Unlike previous mass extinctions (caused by asteroid impacts, volcanic activity, etc.), this one is primarily driven by human activities.

Biodiversity Metrics

Measuring Biodiversity at Different Scales

Biodiversity isn't just about counting species. It's measured at three distinct scales:

• Genetic diversity: The variation in genes within a single species. Higher genetic diversity means a population can better adapt to changing conditions. Cheetahs, for example, have extremely low genetic diversity, making them vulnerable to disease.
• Species diversity: The number and variety of different species in an ecosystem or region. This is what most people think of when they hear "biodiversity."
• Ecosystem diversity: The variety of habitats, biological communities, and ecological processes across a landscape or region. A continent with rainforests, wetlands, deserts, and coral reefs has high ecosystem diversity.

Two key metrics help quantify species diversity:

• Species richness is simply the count of different species in an area. A tropical rainforest might contain thousands of tree species per hectare, while a desert might have only a few dozen plant species total. Richness doesn't tell you anything about how many individuals of each species are present.
• Evenness describes how equally individuals are distributed among species. An ecosystem where five species each make up about 20% of the population has high evenness. An Antarctic penguin colony dominated by a single species has low evenness. A community with both high richness and high evenness has the greatest species diversity.

Ecological Consequences

Importance of Biodiversity for Ecosystem Functioning

Biodiversity underpins the ecosystem services that humans and all other organisms depend on. These services fall into three broad categories:

• Regulating services: Climate regulation, water purification, flood control, pollination, and disease regulation.
• Provisioning services: Food, clean water, timber, fiber, and medicinal compounds. Over 50% of modern pharmaceuticals are derived from natural compounds.
• Cultural services: Recreation, aesthetic enjoyment, spiritual significance, and ecotourism revenue.

Greater biodiversity tends to enhance ecosystem stability (resistance to disturbance) and resilience (ability to recover after disturbance). The mechanism behind this is functional redundancy: when multiple species perform similar ecological roles, the loss of one species is partially compensated by others. Diverse grasslands, for example, maintain productivity during drought far better than monocultures because different species respond differently to water stress.

Keystone species have effects on their ecosystem that are disproportionately large relative to their abundance. Removing a keystone species can trigger a trophic cascade, where changes at one trophic level ripple through the entire food web.

Two classic examples illustrate this:

• Sea otters prey on sea urchins. Where otters have been removed, urchin populations explode and overgraze kelp forests, transforming rich underwater ecosystems into barren "urchin barrens."
• Wolves in Yellowstone: After wolves were reintroduced in 1995, they reduced elk populations and changed elk grazing behavior. Aspen and willow trees recovered along riverbanks, which stabilized stream banks, benefited beaver populations, and increased habitat for songbirds and fish. The loss of apex predators like wolves can lead to overgrazing, habitat degradation, and cascading species declines throughout the ecosystem.